One thing I'll add to this, from my experience as a custom quilt maker, and user.
If you're cold, and sweating, you have inadequate bottom insulation. If you're hot and sweating, you need to vent your bag/quilt.
It's pretty much that simple. Of course, you may be hot and sweating from having an inappropriately warm bag for your conditions, but usually, people figure that out quicker than "I'm freezing but I'm sweating."
Many many of us here carry inadequate bottom insulation in pursuit of arbitrary base weight goals. Next time you're cold, or sweating on the trail, ask yourself where you're cold, and how you're cold, it may be informative.